Recent advances in signal transduction indicate that specificity among signaling pathways in cells in largely determined by compartmentalization, or by the extent or persistence of a signal. Compartmentalization has been suspected for quite some time to play an important role in cellular communication, based largely on descriptive approaches centered around studies of cellular "architecture." However, recent advances on the molecular biology of signal transduction has allowed for a more dynamic approach, in which genetic manipulations can reveal much about these important aspects of cellular regulation. Indeed, there are numerous examples in both yeast and mammalian cells of signaling proteins or domains within these proteins that function primarily to target other molecules to macromolecular complexes, or specific cellular compartments. These difficult issues represent the next major challenge in signal transduction, and will be the focus of this interdisciplinary meeting.